Circadian rhythms are present in species throughout the animal kingdom. In humans, disorders of circadian timing contribute to circadian-based sleep disorders, maladjustment of shift workers and during jet lag, and may contribute to neuropsychiatric disorders including depression and seasonal affective disorder. A transcriptional-translational feedback loop is at the center of the circadian clock mechanism. The known positive elements driving circadian transcription are CLOCK and BMAL1, two basic helix loop helix proteins that dimerize to activate expression of responsive genes. We have recently generated mice with a null mutation of the Clock gene. Unexpectedly, these CLOCK-deficient mice retain circadian rhythmicity in behavior in constant conditions. Our studies will characterize physiological and molecular rhythms in CLOCK-deficient mice, and assess mechanisms of rhythmicity in the absence of CLOCK. In mice with the previously described dominant negative mutation of Clock, the CLOCK-delta19 protein likely disrupts circadian rhythmicity by interfering with the activity of other key bHLH-PAS proteins, indicating that a major circadian transcriptional activator remains to be identified. A major objective of this project will be to identify this apparent second mechanism for transcriptional activation. We will test the hypothesis that NPAS2, a bHLH-PAS transcription factor closely related to CLOCK, can substitute for CLOCK and thus maintain rhythmicity in CLOCK-deficient mice. We will also determine whether BMAL1 is necessary for rhythmicity in the absence of CLOCK, expecting a finding that will enable studies based on assessment of the functional importance of candidate BMAL1-interacting proteins. The proposed studies are necessary to understand the function of CLOCK, a central component of the circadian clock mechanism, and thus are relevant to understanding and possibly developing novel treatments for circadian-based sleep and psychiatric disorders. In addition, the circadian clock plays diverse roles in regulating reproduction, metabolism, cell growth and tumor progression, so the importance of understanding basic mechanisms of circadian rhythm generation has many implications.